When one wishes to attach plastic parts together, several processes exist to accomplish this. Among them are methods such as hot air bonding, sonic welding, vibration welding, adhesives, mechanical fasteners and infrared radiant heat.
U.S. Pat. No. 5,750,970 discloses a method of dielectrically heating an adhesive which, in turn, bonds plastic parts together.
Infrared radiant heat has certain advantages over the other types of processes. Generally, there is a desire to form a strong bond between the two parts quickly at a minimal cost. Additionally, it is preferable that during the bonding process nothing touches the surfaces to be bonded in order to assure uniform bonds from part-to-part and to reduce the cost of cleaning the bonding apparatus. Further, in many instances, it is preferable that no marring or distortion occur on the surface opposite the surface which is bonded. This surface is referred to as the “viewing” or “class A” surface.
Infrared bonding, in general, can overcome many of these concerns. Infrared energy can be finitely pinpointed with a focal point or a mask to the exact area to be bonded in order to avoid overheating the plastic in adjacent areas that might cause unwanted distortion in the part. Infrared energy can heat the bonding surface to a high temperature, thus assuring that a strong bond is formed between the two parts. The bonding surface can be heated very quickly with infrared energy and the timing and amount of heat application can be precisely controlled. Further, with infrared heating, there need be no contact between the heat source and the bonding surfaces of the parts, in order to minimize cleaning requirements for the tooling.
Nonetheless, there are some drawbacks, in general, to using infrared heat to bond plastic parts together. In the case of automotive interior parts, for example, there are many parts made of plastic that are bonded to another part, but also need to have their viewing surfaces as distortion and mar free as possible since occupants of the vehicle will be able to see the surfaces. However, in the general application of focused infrared heat on the bonding surface in such applications, the “class A” surface is oftentimes damaged. The finite focal point, while avoiding overheating adjacent areas, is too intense and harms many plastic parts during the heating process. An inherent difficulty to overcome in working with infrared radiant heat to bond plastic is to heat the bonding surfaces sufficiently without causing warpage, burning or marring of the “class A” surfaces.
U.S. Pat. Nos. 3,383,265; 3,549,451; 4,265,954; 5,151,149; 5,244,525; 5,444,814 and 5,522,954 all disclose the use of infrared energy to bond or join plastic or polymeric materials together. The '954 patent discloses the bonding of plastic interior automatic parts having “class A” surfaces after each surface is exposed to infrared heat from an infrared heat source positioned between the surfaces.
U.S. Pat. No. 4,096,306 discloses a method of forming air-inflated cushioning material using two heat sealable films with air passages therebetween, which may be fused by “infrared black line sealing.”
U.S. Pat. Nos. 5,062,661; 5,487,557; 5,465,998; 5,549,323; 5,558,364; 5,520,412; 5,542,694; 5,399,819; and 5,522,954 disclose a variety of heat-staking and hot plate welding techniques with respect to automotive air bag covers.
Securing back plates to front panels of plastic air bag covers present unique problems especially since the space between the front panel and the back plate defines a critical gap in which a membrane switch is positioned prior to the staking process. The size of the gap is critical to ensure proper switch actuation, which most often takes the form of a horn actuation. Furthermore, the back plate must be secured to the front panel so that the back plate does not separate from the front panel during air bag deployment within an automotive interior temperature range of −30° C. to 80° C.
U.S. Pat. No. 5,685,561 discloses the induction welding of a back plate to the back side of an air bag cover.
U.S. Pat. No. 5,642,901 discloses a relatively flexible thermoplastic air bag cover including a front panel wherein switch-activating members enhance activation of a membrane-type switch located at a switch location area of the front panel.
One prior art staking process used hot air to melt rearwardly extending stakes from the front panel onto the back plate. However, the hot air also tends to: 1) deform the “class A” surface of the front panel; 2) deform the switch membrane due to the hot air flow into the critical gap; and 3) deform the back plate, thereby changing the dimensions of the critical gap.
The use of heat-activated adhesive to bond parts together is shown by the following U.S. Pat. Nos.: 5,228,108; 5,743,982; 5,486,252; 5,858,159; and 5,743,408.